1. Technical Field
This invention relates to manufacturing process controls, and in particular to the controlling of a product manufacturing process to maintain the product parameters within preselected permissible limits.
2. Background Art
In manufacturing processes, including those of forming a plurality of individual products and those wherein a single product is continuously formed, it is desirable to monitor different parameters of the products so as to minimize rejection and scrap. To this end, sensing devices have been developed which provide indications of desired parameter values during the manufacture of the products. Illustratively, where a web is being manufactured from a thick mat by a compression process, the thickness parameter of the produced web may be constantly sensed. Means may be provided for signalling an out-of-tolerance condition so that the manufacturer may make suitable adjustments in the apparatus to bring the thickness parameter back to within the desired limits.
In certain manufacturing processes, feedback controls are provided for automatically adjusting the apparatus as a function of the parameter determinations. A problem arises in the conventional manufacturing process due to the production of random out-of-tolerance products which may be caused to be out of tolerance for a number of reasons, each of which is directed primarily to a single one of the products. Thus, while a series of products may be manufactured in an automatic manufacturing process which are generally within tolerance, every now and then a product is produced which, for some reason, is out of tolerance and is, therefore, caused to be rejected. The conventional automatic production test equipment satisfactorily separates such reject products from the products within the desired tolerance limits and, thus, provides for basic quality control.
However, in conventional manufacturing processes, progressive failures may occur which are not random, but rather, affect the entire production. Illustratively, such progressive failures may be due to gradual dulling of cutting tools, loss of calendar roll preload, introduction of supplies of defective raw material or use of defective subassemblies, etc.
Such nonrandom degradation occurs gradually and over a long period of time and, thus, is difficult to detect and therefore, in many cases, remains uncorrected so as to cause substantial scrap loss. Because of the nature of such nonrandom degradation, it must be detected by statistical methods which heretofore have not been suitable for effecting immediate correction.